The Supreme Court said yesterday that the Environmental Protection Agency may consider whether protecting fish and other aquatic creatures is worth the cost of the most advanced upgrades for older power plants, a defeat for environmentalists who had challenged the government's position.

The court ruled 6 to 3 that such cost-benefit decisions are allowed under the Clean Water Act as the agency moved to require more than 500 older power plants to upgrade the ways they draw water to cool machinery. Water-intake systems kill 3.4 billion fish and shellfish each year, the EPA estimated.

But the technology that could bring the older plants more in line with new plants would cost about $3.5 billion annually, the EPA said.

Environmentalists argued that the Clean Water Act requires remedies that "reflect the best technology available for minimizing adverse environmental impact," and that Congress understood it was nearly impossible to put a monetary value on the loss of wildlife. But Justice Antonin Scalia wrote that even the environmentalists acknowledged that there was some limit to whether the most advanced technology was worth it.

"It seems to us, therefore, that the phrase 'best technology available,' even with the added specification 'for minimizing adverse environmental impact,' does not unambiguously preclude cost-benefit analysis," he wrote.

Power plants draw more than 214 billion gallons of water from U.S. waterways daily to cool machinery, "squashing," in Scalia's words, billions of fish and other small aquatic creatures against intake screens or sucking them into the cooling systems. In newer plants, closed-cooling systems reduce the rate by 98 percent.

But it is extremely costly to implement such systems at older plants, and the EPA said less expensive plans would reduce the loss by 80 to 95 percent.

Industry has long advocated the cost-benefit analysis embraced by the Bush administration's EPA. But whether the Obama administration will feel the same is another question, and environmentalists have begun to lobby for a different approach.

Alex Matthiessen, president of the New York-based environmental group Riverkeeper, which brought the lawsuit, noted that the court did not say that EPA is required to use the cost-benefit analysis, only that it may.

"We are looking forward to working with EPA's new administrator, whom we are confident will agree that the Bush EPA regulations failed to satisfy the Clean Water Act," he said.

Environmental groups pointed out that new EPA Administrator Lisa P. Jackson previously headed the environmental protection agency in New Jersey, one of the states that supported Riverkeeper in the suit. An EPA spokeswoman said the department had no comment on the decision.

Justice John Paul Stevens, in dissent, said that the majority was ignoring the plain language of the statute, and that he found it "puzzling" that the court relied on Congress's silence about whether a cost-benefit analysis was appropriate to decide that it was allowed. He was joined by Justices David H. Souter and Ruth Bader Ginsburg.